Reasons Why People Deviate from the Prevailing Speed

and how a higher limit will change their behavior

By Ben Langlotz

A. 55 in a 55, when the flow is 65. The limit is unreasonably low,
so "the flow" is much faster, but "A" wants to avoid a stressful
encounter with the police, a ticket, and an insurance hike. Other
"A"s don't wish to show disrespect for society's rules.

B. 80 in a 65, when the flow is 70. The flow is slowed by a low
limit and fear of tickets, etc. Enforcement is rare, so the real risk
of a ticket is low due to safety in numbers. A hurried, attentive
motorist "B" is going to be over the limit anyway, so why not speed by
enough to make a real difference in arrival time, B reasons.

C. Frightened 45 mph elderly and decrepit Beetles.

D. Racers and suicidal maniacs.

A and B are the most common reasons, C and D are "too" common, but
easily less than 1% of motorists, so have an insignificant effect on
the number of accidents caused by overtaking/speed differential.

(Note: The average driver will overestimate C and D, since he won't
see the bulk of cars that are around the same speed but never pass by,
but will notice C (passed by all) and D (passing all.) C and D ARE
safety hazards, but better addressed by measures other than low speed
limits.)

Suppose we raise limits to 65 urban/ 75 rural interstate. Motorist A
is now happy to go the limit, along with the bulk of all motorists
(85% at or below the limit.) History shows that we may see about a 2
mph increase in the average speed, but much of this must be coming
from A and his bretheren. (And the few "A"s that can't handle the
limit will keep right, since they no longer have the righteous sense
of being a "citizen safety enforcer.") But will the previous flow go
faster? Consider the case of B.

B drove 80 in a 65, when 2/3 motorists were violators. Now (75 mph
limit), he can avoid the paranoia and distraction of watching out for
officers by keeping it a bit under 80 - about his old speed. Most of
the B types will be happy to "get there" in the same time as before.
Those few who think they can still do 10 or 15 over, like before, will
no longer find friendly convoys of violators. They will see few
officers already occupied with a 74 mph "perp," as before. Now, the
officer will be either enforcing lane discipline, observing impaired
or inattentive drivers, or hiding in a speed trap waiting for the true
sprinters. It won't take long for even most of the remaining speeders
to realize that they aren't getting "lucky" about tickets anymore.
Increased complieance by the bulk of motorists effectively exposes and
makes noncompliance stick out like a sore thumb.most drivers.

But what about C and D? C (our kind but frightened elderly driver)
will no longer find that 1/3 motorists are driving at 55. Now, his 45
or 50 mph speed enjoys little company. Virtually EVERY car is now
flying by at 15-20 mph faster. Dangerous? Yes. But C will
undoubtedly perceive the danger as never before, and may reduce or
eliminate freeway driving. Put another way, the low limits that cause
1/3 of drivers to go 55 are "enabling" those least capable drivers to
remain on high speed highways where they don't belong.

And D? Like the last few stubborn Bs above, he will stick out like a
sore thumb. Remember that the Federal Highway Administration study of
the effects of raising and lowering limits found on interstates raised
to 65, that while speeds in most percentile ranges increased by 0-3
mph, the 99th percentile speed DROPPED. My model may seem optimistic,
but it fits with the facts.